1. Background of the Invention
The invention relates to a half-bridge assembly for switching electrical power, wherein at least two semiconductor switches are connected in series forming a half-bridge. Each semiconductor switch comprises a control input. Furthermore, each first semiconductor switch has a first power terminal which can be connected with a high voltage potential, and each second semiconductor switch comprises a second power terminal which can be connected with a low voltage potential. A second power terminal of each first semiconductor switch is connected with a first power terminal of the respective second semiconductor switch. Finally, each of the semiconductor switches comprises a free-wheeling diode which is located parallel to both power terminals of the respective semiconductor switch.
2. State of the Art
Such a half-bridge assembly is known from DE-A-42 30 510. Such half-bridge arrangements are employed for the formation of inverters for the most different fields of application, such as e.g. for the supply of polyphase machines, permanent magnet motors and the like (see also e.g. DE-A-40 27 969).
In particular with FET power semiconductor switches, the free-wheeling diode is usually integrated with the semiconductor switch. The integrated free-wheeling diode is formed as a silicon junction diode. Such an integrated free-wheeling diode, however, has a relatively long switching time (in the order of 10 ns to several microseconds). This results in that a significant electrical power loss occurs in the integrated free-wheeling diode, which must be converted to heat and dissipated. At least for high electrical switching capacities, half-bridge arrangements are liquid-cooled. Due to the fact that the integrated free-wheeling diodes must possess a considerable current-carrying capacity, these also require considerable semiconductor areas and thus installation space.
In order to reduce the switching times of the diodes and thus the power loss, it has already been proposed to connect Schottky diodes in parallel with the integrated free-wheeling diodesxe2x80x94with the same polarity as the free-wheeling diodesxe2x80x94and to cool each semiconductor switch with its integrated free-wheeling diode and each Schottky diode, i.e. to connect them in a heat conductive manner with a heat sink. For avoiding parasitic inductances, the diodes must be arranged as close as possible to the semiconductor switch.
Schottky diodes have very short switching times, because they are provided with a metal semiconductor transition instead of a pn transition, which also comprises a rectifier effect. With the metal semiconductor transition, however, the stored charge is very small so that the switching time is very short. Another characteristic of Schottky diodes is the lower conducting-state voltage of approx. 0.3 V compared to silicon junction diodes.
The integrated free-wheeling diode is a silicon junction diode with a conduction-state voltage of approx. 0.7 V. Thus, in the free-wheeling operation of the half-bridge arrangement, virtually the total current flows via the Schottky diode because its conducting-state voltage is lower. The Schottky diode has switching times in the order to 10 to 100 ps. Thus the power loss to be converted to heat is also significantly reduced. Nevertheless, the Schottky diode, too, is cooled in the same manner as the power semiconductor switch with the integrated free-wheeling diode, because otherwise an overheating (above 170xc2x0 C.) and thermal destruction would be expected.
This improvement of the switching behaviour also requires a corresponding dimensioning of the Schottky diodes because these must be capable of carrying the total current in the free-wheeling operation of the half-bridge arrangement.
From U.S. Pat. No. 5,661,644 a half-bridge assembly with two semiconductor switches connected in series is known. A free-wheeling diode is connected parallel to each semiconductor switch, and a Schottky diode is connected parallel to each free-wheeling diode. Further, the arrangement of semiconductor switches and free-wheeling diodes on a heat sink is known from this publication.
From DE 195 32 992 A1 a circuit board is known, one side of which is fitted with electric or electronic components, on whose backside a cooling plate is applied under the insertion of an intermediate layer. A component which can be subjected to high thermal loads is connected with the cooling plate via a heat conducting bridge.
From U.S. Pat. No. 6,055,148 a half-bridge assembly with a first semiconductor element arranged on a base plate is known. A Schottky diode is attached to the first semiconductor element by means of an adhesive layer.
From DE 196 24 475 A1 an apparatus for tempering electronic components is known which are arranged on a carrier installed in a housing. A body is arranged between the carrier and a housing wall, which has a closed elastic envelope with a heat conductive material located therein, which is in an essentially air tight contact with the carrier and the housing wall.
Problem on Which the Invention is Based
The invention is based on the problem to further increase the power density (switchable electric power referred to the installation volume) of such half-bridge arrangements and, in particular, to design them even more economic.
Inventive Solution
This problem is solved in a half-bridge arrangement of the above mentioned type in that the thermal resistance between each Schottky diode and the heat sink is dimensioned larger than the thermal resistance between each free-wheeling diode and the heat sink and the thermal coupling of the Schottky diode compared to the thermal coupling of the integrated free-wheeling diode is reduced such that the conducting-state voltage of the integrated free-wheeling diode, even under load, is invariably above the conducting-state voltage of the Schottky diode.
This means that the thermal coupling of the Schottky diode to the cooling medium is xe2x80x9cpoorerxe2x80x9d than the thermal coupling of the integrated free-wheeling diode to the cooling medium.
The xe2x80x9cthermal coupling to a cooling mediumxe2x80x9d in this context can both be the coupling to a (metallic) body and the coupling to a cooling liquid surrounding the semiconductor components.
This measure is contradictory to the usual approach to cool semiconductors as efficiently as possible.
Compared to the state of the art, it nevertheless permits a significantly smaller dimensioning of the Schottky diode with respect to its current-carrying capacity and thus the required semiconductor area because of the following circumstances: The diode characteristic of a Schottky diode is more temperature dependent than the characteristic of an integrated free-wheeling diode with silicon junction diode properties. Moreover, the characteristic of a Schottky diode includes a relatively high ohmic proportion. Upon current supply to the Schottky diode, it is heated to a greater extent because of the poorer cooling so that the conducting-state voltage of the Schottky diode decreases with increasing temperature. The consequence of this is that its characteristic is shifted by a greater amount due to the reduced cooling effect, so that the Schottky diode can take the current from the free-wheeling diode. It is understood that the arrangement as a whole (also with respect to the cooling of the Schottky diode) must be so dimensioned that no destruction of the semiconductors occurs.
If because of the lower heat coupling (and the higher thermal resistance resulting therefrom) between the Schottky diode and the cooling medium the Schottky diode is heated to a greater extent than the integrated free-wheeling diode, the consequence of this is that the conducting-state voltage of the integrated free-wheeling diode, even under load, is invariably above the conducting-state voltage of the Schottky diode. Consequently, in the free-wheeling operation of the half-bridge arrangement, the current always flows through the more rapidly switching Schottky diode.
Advantages of the Invention
Through the inventive construction of the half-bridge assembly a particularly compact arrangement is achieved which enables a packing density which cannot be compared with previous solutions. Compared to previous designs, a reduction of the semiconductor area of the Schottky diodes by approx. 66 percent is possible. This allows to both to keep the required volume of cooling medium or cooling liquid, respectively, small referred to the total volume and to achieve a miniaturisation of the overall arrangement which will render the employment of the invention, in particular also in mobile applications, very economic.
Further Embodiments of the Invention
In a presently preferred embodiment of the invention a ceramic platelet or a synthetic material layer is arranged between the Schottky diode and a cooling body for the Schottky diode, with a copper plating being provided on both sides, if required, while the semiconductor switch with its integrated free-wheeling diode is thermally coupled directly with the cooling body. It is, however, also possible to select a cooling body for the Schottky diode which in fact is directly thermally connected with the Schottky diode (in the same manner as the semiconductor switch with its integrated free-wheeling diode), but itself has a higher thermal resistance against its environment (liquid or air stream) than the cooling body for the semiconductor switch with its integrated free-wheeling diode.
In order to ensure that also in the xe2x80x9cstart-upxe2x80x9d phase of the circuit, i.e. with cold semiconductors and cold cooling medium, the integrated free-wheeling diode of the semiconductor switch cannot be destroyed by a high power current surge, it is advantageous to provide a xe2x80x9cwarm-up phasexe2x80x9d prior to the actual operation of the half-bridge arrangement. During this warm-up phase a current flows through the Schottky diode which, though it results in heating of the Schottky diode and possibly also of the integrated free-wheeling diode, does not lead to its destruction. As soon as the Schottky diode is sufficiently heated, the current, even under full load operation, cannot overload the Schottky diode because its conducting-state voltage will then be sufficiently low.
Due to the fact that for the operation of the half-bridge arrangement or the driving of the consumer connected with the half-bridge arrangement (e.g. asynchronous motor) a control circuit with a control computer is required in order to correspondingly supply the individual phases with current, a program for this warm-up phase can be stored in the control computer. By this control computer program for the warm-up phase the half-bridge arrangement is operated in such a manner that the current through the Schottky diode in the free-wheeling operation remains reduced until the Schottky diode is heated compared to the semiconductor switch and its integrated free-wheeling diode to such an extent that the conducting-state voltage of the integrated free-wheeling diode, even under load, is invariably above the conducting-state voltage of the Schottky diode.
In a preferred embodiment the semiconductor switches are formed by rapidly switching, low-loss field effect transistors (FET""s). Several pairs of semiconductor switches connected in series can be connected in parallel. Moreover, the semiconductor switches can be formed by a great number of individual semiconductor switch elements with small switching capacities each. By utilising numerous semiconductor switch elements with relatively small switching capacities each, but which can be connected in parallel in a simple manner, a good cooling effect can be achieved because the numerous individual components can readily be reached by the cooling medium.
The invention also relates to a power final stage of a control device for a polyphase electric motor, wherein a half-bridge arrangement is provided for each phase of the electric motor.